The present invention generally relates to fuel-fired heating apparatus and, in a preferred embodiment thereof, more particularly relates to heat exchanger systems used in fuel-fired, forced air condensing furnaces.
With the growing need to improve the overall energy efficiency of fuel-fired, forced air heating furnaces, considerable design effort has been directed toward increasing the combustion gas-to-supply air heat transfer capability of their heat exchanger components. Traditionally, fuel-fired furnaces have been designed to extract only sensible heat from the combustion gases generated by their burner systems. This mode of heat transfer is commonly referred to as a "dry" or "non-recuperative" process, and typically provides furnace fuel efficiencies of no more than about 85%.
To capture and utilize otherwise wasted latent combustion gas heat, recuperative or "condensing" type heat exchangers have been used in which a secondary or "wet" heat exchanger is connected in series with the primary or "dry" heat exchanger at its discharge side. During furnace operation, the primary heat exchanger performs its usual task of extracting sensible heat of the combustion gas, and the secondary heat exchanger operates to extract primarily latent heat, thereby considerably lowering the temperature of the combustion gases ultimately discharged to atmosphere, by the operation of a draft inducer fan, via the furnace vent stack.
The use of condensing type primary/secondary heat exchanger systems of this type potentially raises the overall heat exchanger thermal efficiency to about 95% or higher. However, due to the addition of the secondary heat exchanger, the overall size of the high efficiency condensing furnace is correspondingly increased, thereby also undesirably increasing the material cost of the furnace and its outer jacket heat loss. Moreover, the flue gas side and the supply air side pressure drops of the higher efficiency furnace are also increased. This, in turn, usually necessitates the use of a larger draft inducer fan and supply air blower. As a result, the overall noise level and electric power consumption are undesirably increased.
Under conventional practice, gas-fired residential condensing furnaces have typically been provided with "clamshell" type heat exchangers. This type of heat exchanger structure requires a relatively large interior flue gas flow area and its heat transfer rate is relatively low. In order to compensate for this low heat transfer efficiency, the overall size of the typical clamshell heat exchanger tends to be quite large. The large body of the heat exchanger not only increases the overall system cost, but also significantly increases the furnace jacket heat loss due to the large heat flux radiated toward the jacket from the clamshell heat exchanger.
In an attempt to reduce the problems, limitations and disadvantages associated with clamshell heat exchangers, various recuperative heat exchanger designs have utilized serpentined tubular primary heat exchanger sections coupled to condensing secondary heat exchangers. Conventional condensing heat exchanger designs of this type have typically utilized relatively large primary heat exchanger flame tubes (in the range of from 1.75" diameter to 2.5" diameter) due to the belief that smaller diameter tubes unavoidably lead to increased flame quenching by the tube walls, thereby producing reduced efficiency combustion, and that such small diameter flame tubes would generate a highly turbulent internal flame which would lead to unacceptably high combustion noise.
However, when flame tubes in the conventional 1.75"-2.5" diameter range are used in the primary section of a recuperative heat exchanger, the bending radius of the tubes needs to be proportionally increased in order to have a similar manufacturability, thereby leading to an undesirable increase in the heat exchanger system and furnace jacket sizes. Furthermore, due to a relatively low heat transfer rate for these relatively large flame tubes, more surface area is needed to carry out the required heat transfer function. As a result, the size of the tubular heat exchanger system for conventionally designed condensing furnace has tended to be undesirably large.
Also, under conventional heat exchanger design practice, a relatively large number of flame tube passes, typically five to eight, have been used in prior condensing furnaces. Because the flue gas and supply air side pressure drops both increase with the increase tubular passes, the condensing furnace with a tubular heat exchanger system of conventional design typically has a very high inside (flue gas side) and outside (supply air side) pressure resistance and requires a more powerful draft inducer fan and supply air blower. In addition to the increased system cost resulting from this traditional heat exchanger design, the overall noise level of the furnace is increased because the main noise sources in the furnace are fluid moving devices - i.e., the draft inducer fan and the supply air blower.
More importantly, large draft inducer fans and supply air blowers also increase the furnace's electrical power consumption, which is often overlooked in furnace design. According to a recent study, the electrical power consumption of the flow moving devices can be as high as ten percent of the total furnace energy input. Although the electrical power consumption of a furnace is not currently taken into account in determining the overall power efficiency of a furnace, it negatively impacts the furnace operating cost and reduces the operating cost savings advantages otherwise potentially available in enhanced efficiency condensing furnaces.
In view of the foregoing it can be seen that a need exists for a high efficiency fuel-fired condensing furnace, having an improved recuperative heat exchanger, which is more compact, quieter, and has lower flue gas and supply air side pressure losses than conventional condensing furnaces of the type generally described above. It is accordingly an object to provide such a condensing furnace.